Bone loss is a hallmark of inflammatory diseases, including periodontitis and arthritis. Osteoclast-mediated bone and cartilage resorption involves solubilization of the mineral component and proteolytic cleavage of type l collagen and other matrix proteins. A central question in bone biology concerns the nature of the proteolytic enzymes critical to the resorptive process. Our previous studies have demonstrated that gelatinase B (MMP-9) is an abundant protease in human osteoclasts and chondroclasts, which may be responsible for the removal of denatured collagen and possibly other matrix components during bone and cartilage resorption. To investigate the in vivo function of this enzyme in resorption, targeted disruption of the gelatinase B gene will be carried out to produce a mutant mouse (gelB-/-) (Aim 1). The skeletal structure of gelB-/- embryos will be examined in whole mount preparations. Abnormalities in the skeletal development of mutants are expected, since endochondral ossification requires resorption of the cartilaginous model by chondroclasts (Aim 2). The resorptive capacity of osteoclasts in gelB -/- mice will be examined using in vivo and in vitro assay systems, including local IL-1alpha injection and [45Ca] release assays (Aim 3). The importance of gelatinase B for bone and cartilage resorption in inflammatory conditions will be studied using a periapical lesion model, as well as a murine model for rheumatoid arthritis (Aim 4). These studies will provide in vivo models to define the role of gelatinase B in bone/cartilage development and in normal and pathological bone resorption. Identification of gelatinase B as a critical collagenolytic enzyme in resorption would make it an attractive target for therapeutic intervention in inflammatory diseases such as periodontitis and arthritis.